This invention relates to a prosthetic device and more particularly to a prosthetic device comprising an improved knee mechanism.
A prosthetic device must be so designed that the user of the device can walk in a manner as closely as possible to the walking motion of a person with natural limbs, as illustrated in FIG. 1. As can be seen from the figure, walking is divided into two phases: a striding or weight-bearing phase (I) including stages A through H and a swing or non-weight-bearing phase (II) including stages I through M. At the initial stage of the weight-bearing phase (stage A), the body parts (a: neck, b: waist, c: knee) are well behind the foot while the thigh and the shank joinedly connected by the knee form a substantially straight line. As the foot comes into complete contact with the ground and the walking proceeds, the knee flexes (that is, the thigh and the shank bend with respect to each other) as at stages B through E, and then the heel of the foot begins to lift off the ground, as at stages F and G. At the time when the toe of the foot clears the ground (stage H), the striding or weight-bearing phase ends. During the non-weight-bearing phase (II), the walker swings the thigh forward about the groin and the shank forward about the knee (stages I through L) and the weight-bearing phase starts again when the heel of the foot contacts the ground (stage M).
Thus, in order to allow the user to simulate natural walking motion, a prosthetic knee mechanism jointedly connecting a thigh element and a shank element should be so designed that, in the striding or weight-bearing phase, it can absorb the shock produced by the contact of the foot device with the ground and also enable the thigh element and the shank element to bend sufficiently relative to each other while supporting the weight of the user (i.e. without causing knee buckling). Besides, such knee mechanism should be so constructed that the shank element can be swing freely in the non-weight-bearing phase.
A typical conventional prosthetic device including a knee mechanism is shown in FIG. 2. The device includes an element 1 for jointedly connecting a thigh element 2 and a shank element 3. The end portion, toward the back of the user of the device (the left side in the figure), of the joint element 1 is pivotably mounted about a horizontal shaft 4 to which the shank element 3 is secured. The thigh element 2 is pivotably mounted, about a horizontal shaft 5, on the other side (i.e. the front of the user) of the joint element 1. The thigh element 2 may be directly secured to the joint element 1. The joint element 1 is made of a relatively pliable material and is so configured that there is a gap 6 extending from the shaft 4 toward the front of the user, thus separating the joint element 1 into two parts 7 and 8. Between the thigh element and the joint element 1, there is provided a small spacer element 9 secured to the thigh element 2 or the upper surface of the joint element 1. Generally there are also provided a stopper 10 on the joint element 1 and a stopper 11 on the shank element 3.
In the swing or non-weight-bearing phase with such device the shank element 3 can swing forward since the joint element 1 is pivotably mounted about the shaft 4 to which the shank element is secured. The forward swing movements of the thigh element 2 and the shank element 3 are restricted by the stopper 10 and the stopper 11, respectively, so that the thigh element 2 and the shank element 3 form a substantially straight line.
In walking with the prosthetic device to simulate the striding or non-weight-bearing phase, particularly such stages as B through E where the knee flexes, the load of the body weight of the user acts behind the shaft 4 serving as a knee joint, as shown by the arrow in FIG. 2, and hence rotates the thigh element 2 backward (i.e. anti-clockwise in the figure) about the shaft 4, thus lifting the part 7 of the joint element 1 while pressing down the part 8 of the joint element 1 through the spacer element 9. As a result, the element 1 clamps the shaft 4 to which the shank element 3 is secured, so that the thigh element 2 is fastened with respect to the shank element. Thus, the intension is that the prosthetic device can support the body weight of the user, with the thigh element 2 and the shank element 3 being bent relative to each other.
However, such conventional prosthetic device has a drawback in that there is produced only a relatively small moment about the shaft 4 by the joint element 1 to clamp the shaft 4, since said shaft is of a small diameter. Thus, with such device there sometimes occurs knee buckling, i.e. the user sometimes falls backward because of the failure of the device to support the body weight. It may be considered that the clamping force can be increased by enlarging the diameter of the shaft. However, this would rather result in a practical disadvantage since the strength of the joint element 1 would be decreased because of reduction of the width between the shaft and the joint element (d in FIG. 2). The conventional prosthetic device as illustrated is also weak in strength in view of the requirement that the joint element 1 be made of a pliable material to serve as a clamp about the shaft 4. Accordingly, as a matter of fact, the user of such a device must walk in such a manner that the thigh element and the shank element bend as little as possible with respect to each other in order to avoid knee buckling or breakage of the device. Such walking is quite dissimilar to the walking motion of a person with natural limbs as illustrated in FIG. 1. Besides, the conventional prosthetic device as illustrates has less cushioning effect for absorbing the shock from the contact of the device with the ground. This is because the construction of the device limits the provision of a cushioning element to a small element such as the spacer 9.